Symex provides, via the optional symex-ide extension (See Installing Symex), a handful of ways to conveniently leverage major mode features for languages, via short keybindings.
For instance, for some languages (especially Lisp languages), e evaluates the selected expression in a REPL, ? looks up documentation on the selected expression (identifier), and r enters a REPL.
These easy integrations with important coding facilities can help you get quick feedback during development. As generations of Lisp users attest, such tight feedback loops are often the engines of quick progress on any task.
See IDE for the full list of IDE commands and keybindings.